How OnBackButtonPressed works?

Hi,
I'm trying to understand how OnBackButtonPressed works because I set the breakpoint in that method but it's never hit. I have two pages - PageOneView ** and **PageTwoView. In App.cs, I set PageOneView to be a mainpage and wrap it with NavigationPage(). PageOnView has a button to go to PageTwoView.

Here are my code.

App.cs

    public class App : Application
    {
        public App()
        {
            // The root page of your application
            MainPage = new NavigationPage(new PageOneView());
        }
    }

PageOneView.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="NavigationBackButton.Views.PageOneView"
             Title="ONE">
  <StackLayout>
    <Button Text="Go to Page Two" Clicked="Button_OnClicked"/>
    <Label Text="One" VerticalOptions="Center" HorizontalOptions="Center" />
  </StackLayout>

</ContentPage>

PageOneView.xaml.cs

    public partial class PageOneView : ContentPage
    {
        public PageOneView()
        {
            InitializeComponent();
        }

        private void Button_OnClicked(object sender, EventArgs e)
        {
            Navigation.PushAsync(new PageTwoView(), true);
        }

        protected override bool OnBackButtonPressed()
        {
            Debug.WriteLine(">>>> PageOneView:Back button pressed");  //***** Never Hit here *****
            return base.OnBackButtonPressed();
        }
    }

PageTwoView.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="NavigationBackButton.Views.PageTwoView"
             Title="TWO">
  <Label Text="TWO" VerticalOptions="Center" HorizontalOptions="Center" />

</ContentPage>

PageTwoView.xaml.cs

    public partial class PageTwoView : ContentPage
    {
        public PageTwoView()
        {
            InitializeComponent();
        }

        protected override bool OnBackButtonPressed()
        {
            Debug.WriteLine(">>>> PageTwoView:Back button pressed"); //***** Never Hit here *****
            return base.OnBackButtonPressed();
        }
    }

Any suggestions?

Thanks,
Jay